The invention relates to a semiconductor diode laser surrounded by a medium and comprising a semiconductor body having a pn junction which at a sufficiently high current strength in the forward direction can produce coherent electromagnetic radiation in a strip-shaped active region located within a resonant cavity, the resonant cavity being constituted at least over part of its length by a periodical variation in the effective refractive index in the longitudinal direction, while the resonant cavity is limited by surfaces substantially at right angles to the active region, at least one of these surfaces being provided with an antireflection coating.
Such a semiconductor diode laser is described in Philips European Patent Application No. 87201626.6 which was laid open to public inspection under No. 0259919 on March 16, 1988.
Semiconductor diode lasers of various constructions are used in many fields. The resonant cavity can be formed in different ways. In many cases, it is constituted by two parallel extending mirror surfaces, for which mostly cleavage surfaces of the semiconductor crystal are used. By repeated reflection on these mirror surfaces, radiation modes known under the designation Fabry-Perot (FP) modes are produced.
According to another embodiment, the resonant cavity is obtained by a periodical variation of the effective refractive index for the radiation produced along at least part of the length of the resonant cavity. Instead of reflection on mirror surfaces, reflection at a grid (constituted by the said periodical variation of the refractive index) is used. Lasers in which this is the case are designated as DFB (Distributed FeedBack) lasers. They exist in various constructions and are known under the designation "Distributed FeedBack"(DFB) lasers, of which construction the semiconductor diode laser described in the aforementioned European Patent Application is an example, and as "Distributed Bragg Reflection" (DBR) lasers. In this application, for the sake of simplicity they will all be indicated by the designation "DFB" laser.
DFB lasers have, as compared with the aforementioned Fabry-Perot lasers, inter alia the advantage that they can oscillate more readily in a single stable longitudinal oscillation mode (Single Longitudinal Mode or SLM mode) within a large temperature range and with a high output power. This is especially important with the use in optical telecommunication because in the SLM mode the chromatic dispersion is minimal so that the signal can be transported over a large distance without disturbance through the optical glass fiber. Further, DFB lasers can be integrated comparatively readily within an electrooptical monolithic circuit.
However, since in general a DFB laser has at the ends of the active region end faces at right angles to the active layer, Fabry-Perot oscillations can occur between them so that in principle the DFB laser has at least one FP mode besides at least one DFB mode with substantially equal amplification.
It is very difficult to manufacture lasers by means of the usual technologies in which the position of the mirror surfaces is exactly in phase with the period of the grid. Moreover, the usual processes result in a spread of the properties of lasers manufactured within a wafer. An example of such a property in which spread occurs is the position of the mirror surfaces with respect to the grid. As a result, there will be among the lasers manufactured from one wafer a number of multimode lasers or lasers passing from one mode to another mode, while the yield of SLM lasers will be low. The yield moreover also depends upon the so-called KL product, in which L is the length of the resonant cavity and K is equal to .pi.*.DELTA.n/.lambda..sub.b, where .DELTA.n is the amplitude of the refractive index variation and .lambda..sub.b is the Bragg wavelength. With a KL product of 2 to 3, the yield of SLM lasers is, for example, 5 to 10% of the yield of FP lasers. With smaller values of this product, the yield approaches zero.
In order to suppress the FP mode not desired in DFB lasers, various measures have been suggested, among which, as described in the aforementioned European Patent Application, the use of an antireflection coating. Substantially the whole quantity of radiation produced by the laser now emanates at the mirror surface (or the mirror surfaces) and the FP mode is suppressed. A disadvantage of this method is that the line width of the SLM strongly increases. The sensitivity to reflection variations also strongly increases. Due to a larger line width and an increased sensitivity to reflection, use, especially at high modulation frequencies, is limited, which, like the fact that the lasers do not operate in the SLM mode, limits the use within the field of optical telecommunication.